The invention relates to a method of modulating a composite color television signal, comprising a luminance signal and a chrominance signal with a fixed chrominance subcarrier frequency, on a carrier wave, the luminance signal being frequency-modulated on the carrier signal and the chrominance signal being modulated on the carrier wave in such a way that the original composite color television signal can be recovered by a single frequency demodulation of said modulated carrier signal.
The invention is especially of significance for systems in which a color television signal is recorded on a record carrier. In such systems the use of a coding is required which utilizes the limited bandwidth imposed by the record carrier in a most effective manner, while, furthermore, a coding should be used which enables the original color television signal to be recovered in a simple manner during readout of said record carrier.
A modulation method of the type mentioned in the preamble which meets this last-mentioned requirement is the frequency-modulation of the carrier signal by the composite color television wave. If a carrier wave thus modulated is recorded on the record carrier the original composite color television signal can be recovered during reproduction by a single frequency demodulation of said modulated carrier wave, which is obviously beneficial for the simplicity of the reproducing apparatus.
In the case of such a frequency modulation of the carrier wave by the composite color television signal, the chrominance signal gives rise to side-band components which are situated at frequency distances equal to the chrominance subcarrier frequency from those of the carrier wave.
If because of the limited bandwidth of the record carrier a comparatively low frequency is selected for the carrier wave, said side band components may give rise to considerable interference. This may be the case in particular if the second-order lower side band component extends into the negative frequency range and consequently manifests itself as a so-called "folded-back" side band component in the positive frequency range within the frequency band occupied by the frequency-modulated carrier wave. In the reproduced picture this "folded-back" lower side band component then gives rise to an interference effect which is also referred to as moire effect.
In order to avoid this undesired interference effect, the frequency of the carrier wave is generally selected so high that the said second-order lower side band component is situated outside the frequency band occupied by the frequency modulated carrier wave, in particular the first-order side bands.